Alternating current networks and devices are typically designed to operate on sinusoidal 50 Hz or 60 Hz alternating voltages and currents. The voltage and current waveforms in modern power distribution systems are seldom sinusoidal, however, because of the increased use of nonlinear electrical loads such as variable speed motor drives. Such nonlinear loads draw a nonsinusoidal current from a sinusoidal voltage source. This results in a waveform distortion which propagates outward from the nonlinear electrical load into the network. The distorted waveform includes frequency components which are multiples of the original frequency of the electrical power (50 Hz or 60 Hz). These high frequency components are called harmonics. The harmonics are created by the nonlinear electrical load. In 60 Hz power distribution systems, harmonics at 300 HZ, 420 Hz, and 660 Hz and other odd-multiples of frequency are particularly significant. It is desirable to limit the production and propagation of harmonics because harmonics can disrupt and damage sensitive electrical devices drawing power from the power distribution system.
One problem with limiting the production of harmonics is that they are difficult to trace. Their source cannot easily be found and measured. Electric customers such as a factory create harmonic pollution which is spread by the distribution system to other customers. Other customers may experience problems due to the harmonics, but the source of the offending harmonics is difficult to identify. If harmonic current sources could be located and measured, harmonic polluters could be required to reduce their harmonic output to comply with specified limits.
There are many commercially available instruments designed to measure parameters of harmonic disturbances such as the magnitudes of the harmonic currents and voltages. Some such instruments measure the direction of harmonic energy flow, thereby helping to locate the harmonic current source. If the energy flows from the left to the right of the metering point, the harmonic power source is on the left, and vice versa. U.S. Pat. No. 4,667,152 to Hayes et. al., and U.S. Pat. No. 5,508,623 to Heydt et. al. operate according to this principle. This technique, however, has the following major shortcomings:
1) It only detects the net direction of harmonic energy flow. The method cannot determine if there are harmonic current sources on both sides of the metering point. PA0 2) The method is qualitative. It is unable to measure the relative contributions of different harmonic current sources if the sources are located on opposite sides of the metering point. PA0 3) The method detects direction based on net harmonic power. Intervening impedances can result in misleading information about the locations of the harmonic current sources. For example, it is possible for the right side harmonic current source to produce more harmonic energy, but for the net harmonic energy flow to be from left to right. PA0 1) is able to quantitatively measure the strength of a single harmonic pollution source even when there are multiple harmonic current sources connected to the power distribution network; and PA0 2) yields correct measurements of a harmonic current source when there exist unknown impedances between the measurement point and the harmonic current source.
What is needed is a technique for measuring a harmonic energy flow which does not suffer from the above disadvantages and which can accurately identify producers of harmonic pollution.